In recent years, launching optical imaging systems into orbit has become an important field of endeavor. Having optical imaging systems located in orbit has allowed numerous advancements in the exploratory sciences, as well as practical applications such as military defense systems and commercial mapping and navigation systems. In general, these optical imaging systems include a telescope, a detector array and housekeeping subsystems necessary to gather and store the optical data. The optical imaging system is placed in a space borne orbit as a payload attached to a spacecraft. The spacecraft provides the instrument with positional stability, power, and thermal management.
Launching an optical imaging system into space is expensive and, therefore, it is a desirable to have systems with as little mass as possible to reduce launch costs. Unfortunately, most of these optical imaging systems include telescopes that are large and massive, having weights of 500 pounds and more.
Another factor limiting the lifetime of these optical imaging systems is the loss of power in the spacecraft. Therefore, the less power used in operating the optical imaging system, the less drain there is on the power supply of the spacecraft. Less drain on the power supply typically results in a longer lifetime for the imaging system.
In addition to the need for power conservation and efficiency, another important feature of an optical imaging systems is its Field of Regard (hereinafter “FOR”). The FOR is a combination of the individualized scenes that a telescope may see during a specific period. The individualized scenes are referred to as the Field of View (hereinafter “FOV”). It is desirable that these optical imaging systems have a large FOR during as short a period as possible.
Since these large telescopes are mounted on a gimbal to allow freedom of motion, when the telescopes are moved quickly over a scene of interest, the spacecraft experiences vibration that must be compensated for. In general, a control loop system detects the direction of the vibration and provides a signal to compensate for the vibration in an opposite direction. The compensation is accomplished by using primary power that effectively shortens the life time of the instrument.
Another concern with these optical imaging systems is the focal plane of the telescope. Light received by the telescope is focused onto a focal plane array having many detectors, which convert the light into electrical signals. These electrical signals must be transferred from each of the detectors to various signal processors via discrete wires. These wires add unwanted mass and, more importantly, reduce the speed of movement of the telescope and increase signal noise between the focal plane array and the signal processors. The wires must be separated by electrical contacts inside the gimbaled platform in order to allow the two axes of rotation typically provided by gimbaled platform.
Referring now to FIG. 1, there is shown a conventional gimbaled optical imaging system 300. The imaging system includes telescope 310 mounted on gimbaled system 320, where the latter, in turn, is mounted on spacecraft 330. The telescope receives light through aperture 312. The light is focused onto detectors 314 located at the focal plane of telescope 310. The gimbaled system includes two sets of gimbals, 320 and 322. Gimbals 322 allow the telescope to rotate between 0 and 90 degrees on an elevation axis. Gimbals 324, which are attached to gimbals 320, allow telescope 310 to rotate 360 degrees around an azimuth axis. By rotating telescope 310 around the azimuth and elevation axes, a large FOR may be achieved.
The signals from detectors 314 must be transferred to the signal processors (not shown) on spacecraft 330. To accomplish this, many discrete wires carry the signals from the detectors to the signal processors. These wires must pass through many contacts between one gimbaled axis and the other gimbaled axis to permit the two-axes of rotation for the telescope. These contacts are difficult to design and add signal noise to the optical image produced by the image processors.
In addition, the gimbaled movement of telescope 310 requires considerable amounts of energy, since the telescope is very heavy. Furthermore, the system requires considerable amounts of energy to compensate for the vibration effects produced by the motion of the telescope.
One solution that has been proposed to the problem of wires passing between moving gimbals is the use of a Coude path system. In the Coude path system, the detectors of the telescope are not positioned at the back of the telescope, rather they are located in the spacecraft but are away from the telescope. Thus, light received by the telescope is permitted to pass through hollow portions in the gimbals using a series of mirrors in the Coude path system. The light eventually arrives at the focal plane array which is positioned apart from the telescope.
This solution alleviates the problems presented by having wires connecting the detectors and the signal processors. However, this solution does not address the problems created by having to compensate for the vibration and signal noise produced by moving the telescope. Furthermore, the Coude path system reduces the FOV of the telescope and increases the amount of time the telescope requires to scan the FOR.
The present invention, as will be described, provides a system and method of accomplishing the two basic goals for space borne optical imaging systems. The present invention provides for an optical imaging system that requires less inertial corrections and, thereby, allows the system to consume less power. The present invention also eliminates the need for gimbaled platforms, thereby allowing for a lighter spacecraft. At the same time, the present invention does not sacrifice the large FOR or the quality of the final image produced. The present invention is described below.